1. Field of the Invention
The present invention relates to a semiconductor memory device, such as Dynamic Random Access Memory (DRAM), particularly to a pattern layout of sense amplifiers and sense amplifier driving circuits employed in such semiconductor memory device.
2. Description of the Related Art
A pattern layout of sense amplifiers and sense amplifier driving circuits employed in a semiconductor memory device, for example, is disclosed in the co-owned pending patent application of Ser. No. 07/705,341, filed on May 24th, 1991, of which the entire disclosure is incorporated herein by reference.
There is, in FIG. 4 of the above mentioned application (not shown in this specification), disclosed a partial pattern layout of a first sense amplifier including negative channel type MOS (NMOS) transistors 232-1a and 232-1b; a second sense amplifier including positive channel type MOS (PMOS) transistors 233-1a and 233-1b; a first sense amplifier driving circuit including a Vss metal line 245a and a NMOS transistor 238a which provide Ground level voltage (Vss) to a Vss node 232-1c of the first sense amplifier; and a second sense amplifier driving circuit including a Vcc metal line 247a and a NMOS transistor 238c which provide supply level voltage (Vcc, such as 5 volts) to a Vcc node 233-1c of the second sense amplifier.
The Vss node 232-1c consists of three metal-diffusion contacts 417, made of aluminum, which electrically connect a first common node metal line 235 with a common source electrode of the NMOS transistors 232-1a and 232-1b in a negative type diffusion region 415. The Vcc node 233-1c also consists of three metal-diffusion contacts 433, made of aluminum, which electrically connect a second common node metal line 237 with a common source electrode of the PMOS transistors 233-1a and 233-1b in a positive type diffusion region 417.
In order to increase the density of LSI or the capacity of a semiconductor memory device, it is required to reduce the size of not only the region forming memory cells but also the region forming the sense amplifier. It is, however, not easy to reduce the diameter (i.e. the dimension of the horizontal cross section) of the metal-diffusion contact formed in each region forming a sense amplifier since the metal-diffusion contact is relatively deep. As the consequence, it is necessary to reduce the number of the metal-diffusion contacts formed in each region forming a sense amplifier, for example, a single contact per sense amplifier.
However, the above solution, the reduction of the number of the metal-diffusion contacts, would reduce the yield rate of the semiconductor memory device since the unevenness in the contact resistance of the contact, caused by manufacturing process, more seriously affects a sense amplifier so as to prevent the sense amplifier from cooperating with another sense amplifier, coupled to the same bit line pair. If worst comes to worst, the sense amplifier could not work due to the flaw (disconnection) of the reduced number of the metal-diffusion contacts formed therein.